On Thursday, Deputy Attorney General Rod Rosenstein appointed former FBI Director Robert Mueller as a special counsel, tasked with leading the federal government's investigation into Russia's alleged involvement in the 2016 presidential election and the Trump campaign's potential ties to that involvement (Trump has firmly denied those allegations). A unique approach, the use of a special counsel — sometimes called a "special prosecutor" — is far from common. However, the ways that a special counsel has been used before suggest the Russia investigation could go down in U.S. history.
First, some background: A special counsel is essentially an independent, non-government person that is assigned to investigate some matter that the federal government can't — or shouldn't — investigate on its own. In this case, Rosenstein likely appointed a special counsel because the investigation into Russia's alleged involvement in the 2016 presidential election could also probe Trump's affairs. On top of that, Attorney General Jeff Sessions has already recused himself from the probe. Although the president has repeatedly denied all claims of collusion with Russia, even calling the investigation a "witch hunt," it would be more than difficult for the Trump government to justify investigating itself.
Enter, Mueller. The former FBI director knows a thing or two about investigations. Since leaving the bureau, he has played a role in some of the most significant, recent investigations outside of the government's scope.
Now, Mueller will take on another high-profile case. The Trump-Russia probe has been a hot topic practically since the presidential campaigns were still running. From a historical perspective, this makes sense: Special counsels aren't assigned often, but when they are, they investigate some pretty serious cases. Again, Trump has decried the appointment of the special counsel, and denied any Russia collusion allegations.
Archibald Cox and Leon Jaworski were appointed special prosecutors to investigate the Watergate scandal during President Nixon's administration. Later, Lawrence Walsh was appointed to investigate the Iran-Contra affair, which involved the sale of arms and the recovering of American hostages. Even more recently, a special prosecutor, Kenneth Starr, was appointed during the Clinton administration to investigate President Clinton's real-estate investments. Ultimately, Starr began investigating the Monica Lewinsky scandal as well.
From these instances, it would seem that the use of a special counsel inherently signifies that a serious matter is at hand. At this point, it's far too early to tell what Mueller's investigation will reveal (if anything at all), and if it will have any of the same consequences as previous special investigations.